When the accelerator pedal of a vehicle is pressed while the clutch is disengaged, the accelerator (throttle) is opened to cause the engine to run at “idle” and the engine RPM (the number of revolutions of the engine) is stabilized at the engine RPM corresponding to the accelerator opening degree (accelerator position). At that point in time, the driving force generated by the engine and the internal resistance (friction) of the engine are in equilibrium and the engine output torque is 0. That is, the engine does no work for the outside the engine and fuel is wasted. For example, when the engine run at an idle speed of 2000 rpm, the driver hears loud engine noise and therefore can notice that a considerable amount of fuel is being wasted.
The engine does no work for the outside not only during idling caused by disengagement of the clutch as described above but also while the vehicle is in motion. That is, the engine only revolves at an engine RPM corresponding to the accelerator opening degree as in idling and does not contribute to acceleration or deceleration of the vehicle. In this situation, the fuel is consumed only in causing the engine to revolve uselessly, which is highly wasteful.
The present applicant has proposed a coasting control device that performs coasting control (also called fuel-efficient running control) that disengages the clutch and places the engine at idle to save fuel consumption while the engine is running but is doing no work for the outside the engine (Patent Literature 1).